Re: [RFC PATCH 1/3] tracing: Add support for logging data to uncached buffer

[Sai Prakash Ranjan]

On 8/16/2018 8:29 AM, Steven Rostedt wrote:
> Sorry for the late reply, I actually wrote this email over a week ago,
> but never hit send. And the email was pushed back behind other
> windows. :-/

Thanks for the review Steven.
And no problem on late reply, I was working on Will's comment about 
instrumentation in arch code and was about to respin a v2 patch. I have 
replied inline, let me know if any more corrections or improvements can 
be done. I would also like if Kees or someone from pstore could comment 
on patch 2.

> On Fri,  3 Aug 2018 19:58:42 +0530
> Sai Prakash Ranjan <saiprakash.ranjan@xxxxxxxxxxxxxx> wrote:
>
> > diff --git a/kernel/trace/trace_rtb.c b/kernel/trace/trace_rtb.c
> > new file mode 100644
> > index 000000000000..e8c24db71a2d
> > --- /dev/null
> > +++ b/kernel/trace/trace_rtb.c
> > @@ -0,0 +1,160 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/*
> > + * Copyright (C) 2018 The Linux Foundation. All rights reserved.
> > + */
> > +#include <linux/atomic.h>
> > +#include <linux/dma-mapping.h>
> > +#include <linux/export.h>
> > +#include <linux/kernel.h>
> > +#include <linux/module.h>
> > +#include <linux/of_device.h>
> > +#include <linux/platform_device.h>
> > +#include <linux/rtb.h>
> > +#include <linux/sched/clock.h>
> > +
> > +static struct platform_device *rtb_dev;
> > +static atomic_t rtb_idx;
> > +
> > +struct rtb_state {
> > +	struct rtb_layout *rtb;
> > +	phys_addr_t phys;
> > +	unsigned int nentries;
> > +	unsigned int size;
> > +	int enabled;
> > +};
> > +
> > +static struct rtb_state rtb = {
> > +	.enabled = 0,
> > +};
>
> No need for the initialization, you could just have:
>
> static struct rtb_state rtb;
>
> And it will be initialized to all zeros. Or did you do that to document
> that it is not enabled at boot?

I will correct it in v2. RTB will not be enabled until pstore is 
registered since we use pstore for logs. I will add a comment above the 
static declaration saying so.

> > +
> > +static int rtb_panic_notifier(struct notifier_block *this,
> > +					unsigned long event, void *ptr)
> > +{
> > +	rtb.enabled = 0;
> > +	return NOTIFY_DONE;
> > +}
> > +
> > +static struct notifier_block rtb_panic_blk = {
> > +	.notifier_call  = rtb_panic_notifier,
> > +	.priority = INT_MAX,
> > +};
> > +
> > +static void rtb_write_type(const char *log_type,
> > +			struct rtb_layout *start)
> > +{
> > +	start->log_type = log_type;
> > +}
> > +
> > +static void rtb_write_caller(u64 caller, struct rtb_layout *start)
> > +{
> > +	start->caller = caller;
> > +}
> > +
> > +static void rtb_write_data(u64 data, struct rtb_layout *start)
> > +{
> > +	start->data = data;
> > +}
> > +
> > +static void rtb_write_timestamp(struct rtb_layout *start)
> > +{
> > +	start->timestamp = sched_clock();
> > +}
>
> Why have the above static functions? They are not very helpful, and
> appear to be actually confusing. They are used once.

Yes you are right, will remove those.

> > +
> > +static void uncached_logk_pc_idx(const char *log_type, u64 caller,
> > +				u64 data, int idx)
> > +{
> > +	struct rtb_layout *start;
> > +
> > +	start = &rtb.rtb[idx & (rtb.nentries - 1)];
> > +
> > +	rtb_write_type(log_type, start);
> > +	rtb_write_caller(caller, start);
> > +	rtb_write_data(data, start);
> > +	rtb_write_timestamp(start);
>
> How is the above better than:
>
> 	start->log_type = log_type;
> 	start->caller = caller;
> 	start->data = data;
>   	start->timestamp = sched_clock();
>
> ??

Sure, will change it to above and post v2.

> > +	/* Make sure data is written */
> > +	mb();
> > +}
> > +
> > +static int rtb_get_idx(void)
> > +{
> > +	int i, offset;
> > +
> > +	i = atomic_inc_return(&rtb_idx);
> > +	i--;
> > +
> > +	/* Check if index has wrapped around */
> > +	offset = (i & (rtb.nentries - 1)) -
> > +		 ((i - 1) & (rtb.nentries - 1));
> > +	if (offset < 0) {
> > +		i = atomic_inc_return(&rtb_idx);
> > +		i--;
> > +	}
> > +
> > +	return i;
> > +}
> > +
> > +noinline void notrace uncached_logk(const char *log_type, void *data)
>
> BTW, all files in this directory have their functions notrace by
> default.

Oh I missed it. Will remove notrace.

- Sai Prakash